Bicharge photoconductive zinc oxides containing iodine

ABSTRACT

Bicharge photoconductive zinc oxides useful in the production of photoprint papers providing good quality photoprints in both positive charge and reversal modes are prepared by heating photoconductive zinc oxide in air to a temperature of from about 100 DEG C. to about 500 DEG C. in the presence of iodine or an inorganic or organic iodide.

This invention relates to bicharge photoconductive zinc oxides andmethods of making them. Unlike ordinary photoconductive zinc oxideswhich perform well in negative charge photocopying, a bicharge zincoxide can accept and discharge both negative and positive charges in acontrolled manner which affords the production of either negative orpositive photocopy. This dual capability adds much flexibility to thepractical use of photocopy papers coated with the photoconductive zincoxide. The bicharge capability is particularly useful and desirable inthe microfilm copying art.

Commercially available bicharge zinc oxides are off-colored and producelow quality, speckled photoprints. The poor quality reproduction occursin the positive charge mode (positive corona surface charge,subsequently developed with a negatively charged toner) and in reversalphotocopying. The reversal process is particularly useful in copyingmicrofilm where the negative microfilm is enlarged and reproduced as apositive print. This is accomplished by placing the photoimage, by lightreflection, onto a negatively charged photoconductive surface.Subsequent development with a negatively charged toner yields thedesired and conventional positive print.

I have discovered a simple, direct process for converting ordinaryphotoconductive zinc oxide into a white (i.e., colorless) bichargephotoconductive material which, when coated on photoprint paper,provides good quality photoprints in both positive charge and reversalmodes. In this process elemental iodine, or organic or inorganiciodides, are mixed with the zinc oxide. The mixture, containing 0.005 to0.1 weight percent of iodine (or its equivalent in iodide form), isheated in air at 100-500°C. The residence time is selected to distributethe iodine or iodide uniformly throughout the mass of zinc oxide. In apreferred embodiment zinc oxide mixed with 0.02-0.05 weight percent ofiodine, or the equivalent in iodide form, is heated in static air at200-400°C. for about 10-30 minutes. The resulting bicharge product issomewhat whiter than the starting zinc oxide, and is free flowing andodorless. A similar product is obtained by heating zinc oxide in anatmosphere of gently moving air containing an equivalent quantity ofiodine or of a volatile iodide from an upstream source.

An effective alternative for adding the iodine is to dissolve it in asolvent such as water or carbon tetrachloride in which the zinc oxide issuspended. The resulting slurry is filtered, dried, and then heated inair to yield a bicharge photoconductive zinc oxide.

As mentioned above, the iodine may be provided as the element, or as anorganic or inorganic iodide, including hydrogen iodide. Practically allof the iodine is retained when elemental iodine or inorganic iodides areused. Some iodine loss occurs when volatile organic iodides such asmethyl and ethyl iodide are employed.

High quality positive charge and reversal photoprints require highpositive charge acceptance (i.e., high positive charge saturationvoltage) and decreased rate of dark decay of the acquired positivecharge. The data in Table I show that the best bicharge photopropertiesoccur in the 0.01-0.03 weight percent iodine range where the positivesaturation voltage is considerably higher than that of the untreatedzinc oxide, and the rate of positive dark charge decay is considerablylower. This combination produces the contrast necessary to provide goodquality positive charge and reversal photoprints. Table I also shows therelationship between bicharge photoprint quality and rate of dark decayof the positive charge in volts per second. The quality of thephotoprints was scaled, by visual inspection, as follows: 6 = very poor;5 = poor; 4 = fair; 3 = acceptable; 2 = moderately good; 1 = good. Thescaling was based on print brightness, cleanliness, contrast, andspeckling. For commercial practice, the print quality numerical ratingsshould be less than 4.

                                      TABLE I                                     __________________________________________________________________________    Bicharge Photoproperties and Print Quality of                                 Papers Coated with Iodine-Treated PC Zinc Oxide.                              Effects of Iodine Concentration. All Samples                                  Heated in Air at 400°C for 20 Minutes.                                 __________________________________________________________________________                          Dark Photoprint Quality                                                       Decay                                                                              Reversal                                                                            Positive                                                     Saturation                                                                          Rate (-charge)                                                                           (+charge)                                    Sample                                                                            Wt.% Iodine                                                                           Charge                                                                            Voltage                                                                             V/Sec                                                                              (-toner)                                                                            (-toner)                                     __________________________________________________________________________    1   --      -   875   14.5 6                                                      --      +   760   19.0       6                                            2   0.003   -   850   12.5 5                                                              +   785   17.5       5                                            3   0.01    -   870   14.5 2                                                              +   740   12.5       2                                            4   0.03    -   910   15.5 1                                                              +   810   12.0       1                                            5   0.06    -   885   20.5 3                                                              +   800   14.0       3                                            6   0.12    -   920   20.0 4                                                              +   835   17.0       4                                            7   0.30    -   935   22.5 4                                                              +   850   17.0       4                                            __________________________________________________________________________

The data in Table II show that whereas bicharge print quality, incomparison with the untreated zinc oxide, is noticeably improved byheating the iodine-zinc oxide admixture over a wire temperature range of100-500°C., the preferred heating range is 100-400°C.

                  TABLE II                                                        ______________________________________                                        Photoprint Quality of Bicharge Paapers Coated                                 with Iodine-Treated PC Zinc Oxide. Effect of                                  Heating Temperature of Iodine-Zinc Oxide Mix-                                 tures at 0.03 wt. % Iodine.                                                   ______________________________________                                                         Photoprint Quality                                                  Temp. °C of                                                                            Reversal    Positive                                          Heat Treatment  (-charge)   (+charge)                                  Sample.                                                                              20 Minutes      (-toner)    (-toner)                                   ______________________________________                                         8     Room temp. (28°C)                                                                      4           5                                           9     100             2           1                                          10     200             1           1                                          11     300             2           2                                          12     400             3           3                                          13     500             4           4                                          14     600             5           5                                          ______________________________________                                    

Iodine and the iodides are uniquely effective for conferring bichargephotoproperties on normal photoconductive (PC) zinc oxide. As shown inTable III, of the four halogens, iodine alone provides good qualitybicharge photoprints. Those prepared from bromine, chlorine orfluorine-treated photoconductive zinc oxide give photoprints ofunacceptable quality.

                                      TABLE III                                   __________________________________________________________________________    Bicharge Photoproperties and Print Quality of                                 Papers Coated with Halogen-Treated PC Zinc Oxide.                             Halogens Added at 0.013 Mole % Level (Equivalent                              to 0.04 wt. % Iodine). Samples Heated in Air                                  at 420°C.                                                              __________________________________________________________________________                         Dark                                                                              Photoprint Quality                                            Charge to   Decay                                                                             Reversal                                                                            Positive                                           Halogen                                                                            Coated                                                                              Saturation                                                                          Rate                                                                              (-charge)                                                                           (+charge)                                      Sample                                                                            Added                                                                              Paper Voltage                                                                             V/Sec                                                                             (-toner)                                                                            (-toner)                                       __________________________________________________________________________    15  Iodine                                                                             -     950   22.0                                                                              1                                                             +     905   16.5      1                                              16  Bromine                                                                            -     930   14.0                                                                              5                                                             +     805   22.5      5                                              17  Chlorine                                                                           -     960   13.0                                                                              6                                                             +     855   21.0      6                                              18  Fluorine                                                                           -     940   17.0                                                                              4                                                             +     790   24.5      4                                              __________________________________________________________________________

The data in Table IV show that the iodine can be applied by suspendingthe zinc oxide in a water or carbon tetrachloride solution of theiodine. The samples were filtered, then air-dried at 110°C. The waterslurry process alone, without iodine, produces some improvement inbicharge print properties. However, the addition of iodine is requiredto obtain acceptably good quality bicharge photoprints.

                                      TABLE IV                                    __________________________________________________________________________    Bicharge Photoproperties and Print Quality of Papers                          Coated with Iodine-Treated PC Zinc Oxide. Samples                             Slurried in Solvent, Then Filtered and Air-Dried                              at 110°C.                                                              __________________________________________________________________________                                  Dark                                                                              Photoprint Quality                                                        Decay                                                                             Reversal                                                                            Positive                                                      Saturation                                                                          Rate                                                                              (-charge)                                                                           (+charge                              Sample                                                                            Wt. % Iodine                                                                          Solvent Charge                                                                            Voltage                                                                             V/Sec                                                                             (-toner)                                                                            (-toner                               __________________________________________________________________________    19  --      Water   -   700   12.5                                                                              4                                                               +   605   24.0      4                                     20  0.02    Water   -   805   18.0                                                                              3                                                               +   680   17.5      1                                     21  0.02    Carbon  -   820   21.0                                                                              2                                                       Tetrachloride                                                                         +   765   15.5      2                                     __________________________________________________________________________

The improvements obtained with a number of organic iodides are given inTable V. In particular, iodoform and ethyl iodide are as effective asiodine in this method.

                                      TABLE V                                     __________________________________________________________________________    Bicharge Photoproperties and Print Quality of Papers                          Coated with PC Zinc Oxides Treated with Organic                               Halides. Samples Heated in Air at 400°C.                               __________________________________________________________________________                                 Dark                                                                              Photoprint Quality                                        Wt. % as        Decay                                                                             Reversal                                                                            Positive                                            Iodine or Saturation                                                                          Rate                                                                              (-charge)                                                                           (+charge                               Sample                                                                            Additive Equivalent                                                                          Charge                                                                            Voltage                                                                             V/Sec                                                                             (-toner)                                                                            (-toner                                __________________________________________________________________________    22  Iodine   0.04  -   930   20.0                                                                              1                                                               +   885   17.5      3                                      23   Methyl Iodide                                                                         0.04  -   945   21.5                                                                              2                                                               +   865   16.5      4                                      24  Ethyl Iodide                                                                           0.04  -   985   22.5                                                                              1                                                               +   890   16.0      2                                      25  Iodobenzene                                                                            0.08  -   970   22.0                                                                              2                                                               +   910   14.0      3                                      26   p-Diiodobenzene                                                                       0.04  -   985   23.5                                                                              2                                                               +   870   18.0      4                                      27  Iodoform 0.04  -   910   22.0                                                                              1                                                               +   845   15.5      1                                      28  Iodoform 0.08  -   985   29.0                                                                              2                                                               +   895   15.0      1                                      __________________________________________________________________________

The data in Table VI show that inorganic iodides, such as those ofpotassium, zinc, aluminum, and tin, produce beneficial bicharge effects.The samples were heated in air at 400°C.

                                      TABLE VI                                    __________________________________________________________________________                                  Dark                                                                              Photoprint Quality                                        Wt. % as        Decay                                                                             Reversal                                                                            Positive                                            Iodine or Saturation                                                                          Rate                                                                              (-charge)                                                                           (+charge)                             Sample                                                                            Additive  Equivalent                                                                          Charge                                                                            Voltage                                                                             V/Sec                                                                             (-toner)                                                                            (-toner)                              __________________________________________________________________________    29  Stannic Iodide                                                                          0.04  -   915   21.0                                                                              3                                                               +   855   14.5      3                                     30  Potassium Iodide                                                                        0.04  -   890   20.0                                                                              1                                                               +   790   14.0      5                                     31  Zinc Iodide                                                                             0.04  -   878   16.0                                                                              1                                                               +   790   12.0      3                                     32  Aluminum Iodide                                                                         0.04  -   860   11.0                                                                              3                                                               +   850   17.0      3                                     __________________________________________________________________________

The following are illustrative examples of the conversion of zinc oxideto bicharge zinc oxide:

Example 1

To a 120 g. sample of conventional photoconductive zinc oxide (Frenchprocess zinc oxide prepared by the combustion in air of purified zincvapor), having an average particle size of 0.22 μ, were added 36 mg.(0.03 wt.%) of pulverized iodine. The mixture was shaken and tumbledperiodically for several minutes, then heated in a covered pyrex dish at400°C for about 20 minutes. The relatively high vapor pressure of iodineensures a uniform distribution of iodine throughout the mass of zincoxide. Analysis of the white product showed essentially quantitativeretention of the iodine.

EXAMPLE 2

To a slurry of 160 g. of conventional photoconductive zinc oxide in 350ml. carbon tetrachloride were added 32 mg. (0.02 wt.%) iodine dissolvedin 50 ml. of carbon tetrachloride. After stirring for 15 minutes, theslurry was filtered, and the cake was dried in air at 110°C for onehour, then pulverized. The product was white.

EXAMPLE 3

To a slurry of 160 g. of conventional photoconductive zinc oxide in 300ml. water were added 32 mg. (0.02 wt.%) of iodine dissolved in 200 ml.water. After stirring for 15 minutes, the slurry was filtered, and thecake was dried in air at 100 °C for 2 hours, then pulverized. Theproduct was white.

EXAMPLE 4

To 160 g. of conventional photoconductive zinc oxide were added 66 mg.of pulverized iodoform (CHI₃). The mixture, after tumbling, was heatedin a covered glass dish at 400°C for about 20 minutes. The product waswhite.

EXAMPLE 5

To 200 g. of conventional photoconductive zinc oxide were added 135 mg.methyliodide (liquid, equivalent to 0.06 wt.% iodine). The mixture wastumbled for several minutes, then heated in a covered glass dish at200°C for about 20 minutes. The white product contained 0.037 weightpercent of iodine.

EXAMPLE 6

To 160 g. of conventional photoconductive zinc oxide were added 84 mg.pulverized potassium iodide (equivalent to 0.04 wt.% iodine). Themixture, after tumbling, was heated in a covered glass dish at 400°C forabout 20 minutes. The product was white.

For testing the bicharge photoconductive zinc oxides were applied to aconductive base paper (Weyerhaeuser Base M) at a coating weight of 20pounds per 3000 square feet. The coating mixture was composed of:

    Zinc oxide                 140 g.                                             Modified Acrylic resin (DeSoto E-041,                                         45% non-volatile solids)   44 g.                                              Toluene                    110 g.                                             Dye Sensitizer; solution of 7.5 mg.                                           Bromophenol Blue and 7.5 mg. Uranine                                          in 6 ml. methanol                                                         

The coated papers were dark-adapted overnight. Electrical measurementson small samples of the coated papers were made on an M/K Stati Tester.The corona was charged to 6000 volts with a current flow of 25microamperes in both negative and positive mode. Exposure to the coronacharge was for about one second, after which the sample was retained ina dark chamber for 10 seconds to measure the rate of charge decay.

The photoprints were made and developed in an SCM Copier, Model 33 inwhich 6000 volts at 25 microamperes were applied to the corona unit. Thepapers were charged for 11/2 seconds, then imaged by exposure to 40footcandles of light reflected from the master print for 11/2 seconds.The reversal prints were made by charging with a negative corona, thendeveloping with a negatively charged toner (Clopay RSX-117). Thepositive prints were made by charging with a positive corona, thendeveloping with the negatively charged toner.

The iodine compounds, other than elemental iodine, which are effectivein the method of the invention include both organic and inorganiciodides such as metal iodides, hydrogen iodide, alkyl and aryl mono andpolyiodides.

I claim:
 1. A method of producing bicharge photoconductive zinc oxidewhich comprises heating photoconductive zinc oxide in air to atemperature of from about 100°C to about 500°C. in the presence ofiodine in an amount of from about 0.005 to about 0.1 percent by weightof the zinc oxide or an inorganic or organic iodide containing anequivalent amount of iodine for a time sufficient to distributeuniformly said iodine or iodide throughout the mass of said zinc oxide.2. A method as defined in claim 1 wherein the time of heating is fromabout 10 to about 30 minutes.
 3. A method as defined in claim 1 whereiniodine or inorganic or organic iodide is contacted with the zinc oxideadmixed in a stream of air.
 4. A method as defined in claim 1 whereinthe zinc oxide is admixed with a solution or suspension of iodine orinorganic or organic iodide is mixed with the zinc oxide before heatingin the presence of air.
 5. Bicharge photoconductive zinc oxide producedby the process of claim 1.